It is often desired to display a movie originally recorded on film by means of a television broadcast. In order to comply with television broadcast field rates, some sort of film-to-video conversion must be performed.
Typically, a movie is recorded and displayed at a frame rate of 24 frames per second. However, television broadcasts use a different rate, such as the 59.94 field per second rate of the NTSC standard where every two fields are interlaced and comprise one frame.
One method of converting film rates to television field rates is referred to as the "3:2 pulldown" scanning method. A first film frame is scanned twice, then a second film frame is scanned three times, the next frame twice, etc. To accommodate the fact that the NTSC vertical scan period is slightly less than 60 fields per second, the actual display rate may be slowed slightly.
At the receiving end of television broadcasts, a recent development has been the conversion of the incoming television signal into digital data for processing. The processing includes compensation to overcome the visual artifacts perceived by the viewer as a result of motion in the scene being displayed. Although various processing techniques have been developed for compensating the effects of motion between interlaced television fields, these methods are not designed for film-to-video formats. It is therefore necessary to detect when an incoming signal has a 3:2 pulldown format so that appropriate motion compensation processing can be performed.